I propose to investigate the role of calcium and cyclic nucleotides in the physiology of the vertebrate rod photoreceptor. It is well established that these substances are involved in generating the rod's electrical response to light. However, the difficulty of observing the light regulation of calcium and cyclic GMP in the intact rod has precluded a clear understanding of their functional roles. I will address this problem by measuring light evoked changes in extracellular calcium concentration in the space surrounding the rod outer segments, utilizing a planar calcium-sensitive membrane electrode. Extracellular calcium measurements provide the only method, at present, for studying the light regulation of calcium in the intact cell. Specifically, I will investigate the relationship between changes in extracellular calcium and the rod's electrical response, to infer the role that intracellular calcium and the rod's electrical response, to infer the role that intracellular calcium plays in generating this reponse. I will investigate these phenomena during light and dark adaptation in order to learn the role of calcium in adaptation. Finally, I will study the effects of altered cyclic GMP levels on the extracellular calcium changes and electrical response to infer the role that cyclic GMP has in the cellular mechanisms of visual transduction and adaptation in the retina.